Immune evasion is the central mechanism facilitating the establishment of persistent HIV infection. The HIV envelope is responsible for virus evasion of the humeral immune response. Functional interaction of the variable loops in gp120 is believed to be the main reason that HIV primary isolates are relatively resistant to neutralization. However, whether there is a direct physical interaction of the V regions as well as the involvement of the constant regions in this interaction are unknown. Our long-term goal is to understand the mechanism of immune evasion and to develop an effective strategy against the virus. As a step toward that goal, the objective of this application is to study the interaction between the variable and constant regions of gp120, and determine the role of this interaction in HIV-1 neutralization. The central hypothesis of this study is that V1/V2 and V3 can physically interact with each other and that, the bridging sheet in the constant region of gp120 modulates. Moreover, this interaction has a direct role in HIV-1 neutralization and entry. This hypothesis will be tested and the objectives will be accomplished by pursuing four specific aims: 1) Determine the interaction between V1/V2 and V3 in primary and laboratory adapted strains, and the correlation of this interaction with the neutralization sensitivity, 2) Determine the role of the bridging sheet of gp120 in the interaction between the V regions and neutralization sensitivity, 3) Determine the role of the dynamic interaction of V1, V2/V3, and the bridging sheet in HIV-1 entry, and 4) Induce neutralizing antibodies directed to the epitopes associated with the binding sheet. The proposed study explores two novel aspects regarding how HIV-1 evades humeral immune response: 1) a direct domain-domain interaction in gp120 and 2) the constant region as a regulator in HIV-1 neutralization sensitivity. We will take advantage of the HIV-1 molecular clones that exhibit a substantial difference in neutralization sensitivity and recombinant proteins that mimic the structural motifs of gp120 to achieve the goals of proposed study. The results of this study are expected to provide new insights into the molecular mechanism of neutralization evasion and viral entry. In addition, the results could provide a new target for anti-HIV-1 interventions including drug and vaccine development.